The present invention relates to a pneumatically operated power tool, such as a pneumatically operated screw driver driven by compressed air to perform a prescribed operation.
Pneumatically operated screw drivers are well known in the art as a type of pneumatically operated power tool. In the examples of Japanese Patent Application Publications Nos. H11-300639 and 2005-118895, the screw driver includes a rotating body driven to rotate by a pneumatic motor, a rotation slide member accommodated in the rotating body so as to be capable of sliding up and down therein, a driver bit mounted on the lower end of the rotation slide member, and a piston formed circumferentially around the lower end of the rotation slide member and fitted into a cylinder so as to be capable of moving vertically therein.
With this type of screw driver, the rotation of the pneumatic motor is transmitted to the driver bit through the rotation slide member, and air compression applied to the piston moves the rotation slide member within the cylinder, thereby applying rotational and axial movement to the driver bit mounted on the rotation slide member in order to drive a screw into a workpiece. After the screw driving operation is completed, compressed air accumulated in a return chamber returns the rotation slide member and the driver bit to their initial states.
Although this screw driver is applied to applications for fastening a gypsum plaster board, for example, to a base member formed of wood, a steel plate, or the like, the amount of energy required for driving the screw in the case of the steel plate varies considerably depending on the thickness and hardness of the steel plate. If the steel plate is considerably thick or hard, the screw driver cannot drive the screw into the plate, as the tip of the screw does not penetrate the plate in some cases. Hence, the pressure of the supplied compressed air is set sufficiently high to produce a large driving force for penetrating the steel plate. However, since this driving force is too large when driving a screw into a thinner steel plate, the screw will penetrate the steel plate too far so that the gypsum plaster board or the like is not securely fastened. Hence, this conventional screw driver requires means for adjusting the force of the compressed air to suit the type of base member.
Conventionally, a pressure reduction valve has been used to change the force of compressed air. Normally, the pressure reduction valve is mounted on or disposed near the compressor at a position separated from the working position. Therefore, the operator of the screw driver must walk to the location, in which the compressor is positioned, to change the pressure reduction valve when the type of base member requires a different driving force, resulting in cumbersome work for the operator.
Hence, some screw drivers that are now available commercially incorporate a pressure changing mechanism having a pressure reduction valve in the body of the screw driver.